Topic 4 Bonding. SL + HL Ionicbond Covalent bond Intermolecular forces Hydrogen bond Dipole-dipole attraction van der Waals’ forces Metallic bond
4.1 Ionic bond Ions = chargedparticles Ionic bond= the electrostatic bond between positively and negatively charged ions Ionic compound= a compound built from ions, i.e. a salt
Atoms want to get a “Noble gas electron configuration”. One way to get that structure is to throw away the valence electron or steal some electrons to get a full outer shell.
Sodium gives one electron to chlorine and both will have noble gas configuration. Which noble gas? Sodium ion: positive electric charge. Cation. Chlorideion: negative electric charge. Anion.
Ioniccrystals • + charge particles and – charge particle attracts each other in three dimension and builds up a lattice/crystal. Strong electrostatic forces in three dimensions. Each cation is surrounded by anions and vice versa.
Ions • Group 1: H+, Li+, Na+, K+, Rb+, Cs+, Fr+ • Group 2: Be2+, Mg2+, Ca2+, Sr2+, Ba2+ • Group 3?/13: B3+, Al3+, Ga3+ • Group 6?/16: O2-, S2-, • Group 7?/17: F-, Cl-, Br-, I-
Naming compounds • Positive ions have the name of the atom: Sodium ion • Negative ions have the name of the atom (or almost) + the ending –ide: Chloride ion • Sodium + chlorine Sodium chloride • Lithium + oxygen Lithium oxide
Formula ofioniccompounds Write the chemicalformulaof the compoundsformedbetween the positive and negative ionsabove. Write the nameof the ioniccompounds
Transition metals • Transition metals can often form more than one ion, e.g.: • Fe2+ and Fe3+ • Cu+ and Cu2+ • HL: colouredbecauseof d-orbitals
Some common polyatomic ions • Nitrate NO3- • Hydroxide OH- • Sulphate SO42- • Carbonate CO32- • Hydrogen carbonate HCO3- (Bicarbonate) • Phosphate PO43- • Ammonium NH4+ Ca(OH)2
Formula ofioniccompounds-2 Write the chemicalformulaof the compoundsformedbetweenthe positive and negative ionsabove. Writethe nameof the ioniccompounds
When can we expect an Ionic bond? • The quick rule: Metal + non-metal => Ionic compounds (Salts) CuSO4 (s)
Electronegativity • Electronegativity is a measure of an atoms power to attract electrons. • On the right side in the periodic table (group 7,6,5) the atoms have high values, they attract electrons readily. Best is Fluorine, e-neg 4. • On the left side the values are low. Low ability to attract electrons.
Ionic bond or not- calculate difference in electronegativity • If you want to do a more “precise” estimation you can calculate it with the help of electronegativity values. • If the difference in electronegativity > 1,7 then you can say it's an Ionic bond.
Ionic bond or not?Use electronegativityvalues in the Chemistry Data booklet • NaCl • MgO • AlCl3 • SiO2 • Ca3N2
Typical properties of Salts Hard, brittle, • Conduct electricity in solution or melted • High melting points => Strong bond • Hydration of Ion in Water solution
4.2 Covalent bond Electron pair bond, molecular bond • If the De-neg < 1,7 the bond is considered to be covalent • Often between non-metals • Polar or non-polar
In a covalent bond the atoms share electrons with each other to get a “Noble gas electron configuration” • The bond has a direction (one atom to an other atom) • One bond consists of two electrons, an electron pair O H H
Single bond: the two atoms share two electrons (1 pair) Double bond: the two atoms share four electrons (2 pairs) Triple bond: the two atoms share six electrons (3 pairs)
Lewis structures • all valence electrons marked by dots or lines. Draw Lewis structures for: • F2 • NH3 • CO2 • N2 • C2H4
Number of bonds bond lengths and bond strengths • As the number of shared electrons increase (single to triple) the bond lengths shortens and the bond energy increase
Non-polar covalent bond • In, for example, H2 the two electrons in the bond are shared equally between the two hydrogen atoms • H-H De-neg =0 • The electron distribution is symmetrical
Polar covalent bond • If two different atoms form a covalent bond there will be a difference in De-neg • The atom with highest electronegativity will have the electrons closer; they don’t share equally • Unsymmetrical electron distribution
Whichmoleculescontains at leastone polar covalent bond? F2 HF NF3 SiF4 • Which bond in the pairs belowhave the highest polar character? • C-O, N-O • H-O, S-O • H-O, H-S • Se-S, Se-F
Ionic, polar or non-polar covalent bond? • % ionic character of a bond: 0-90% (there are no 100% ionic compounds) H-Cl Na+Cl- Cl-Cl
Dative covalent bond • In a “normal” covalent bond both atoms contribute with electrons to the bond. • Sometimes only one atom contributes with both electrons (the electron pair) to the bond • Then the covalent bond is called a dative covalent bond
Examples of dative bonds • H2O + H+ H3O+ • NH3 + H+ NH4+ • C (4 ve-)+ O (6 ve-) CO
VSEPR • Valence Shell Electron Pair Repulsion • Shape and bond angles • Determine the molecules structure, the shape in 3 dimensions • Structure around a given atom is determined principally by minimising electron-pair repulsion • Bonding or non-bonding pairs will be as far apart as they can.
Linear • Two negative centers • 180o • E.g. CO2
Trigonal planar (flat) • Three negative centers • 120o • E.g. BF3
Tetrahedral arrangement • Four negative centers • 109.5 o
Method • Draw Lewis structure • Count electron pairs, minimise the repulsion • Positions of the atoms • Name of the structure
Ammonia, NH3 Tetrahedral, Trigonalplanar 107o, one non-bonding electron pair • Non-bonding pair (lone pair) takes more space => reduce bond angels
Water, H2OTetrahedral, Non-linear (bent) 104oTwo non-bonding electron pairs Non-bonding pair (lone pair) takes more space => reduces bond angles
Non-polar and polarmolecules (dipoles) • Based on bond polarity and molecular shape • May predict how a molecule will behave with other compounds • Polar molecule = Dipole
A polar molecule (a dipole) • Must have polar covalent bonds. • Look at the difference in electronegativity. FONClBrISCH AND • Unsymmetrical shape according to charge distribution. • Otherwise it will be a non-polar molecule (NOT a dipole)
Dipole or not? • H2O • CO2 • NH3 • CH4 • CH3OH
Allotropes • Some elements can be found in different forms • E.g: • Carbon: Diamond, graphite, C60fullerene • Oxygen, Ozone See PPT: Carbonallotropes
Silicon • Metalloid, Semiconductors, non-metallic structure • Similar structure as diamond
Silicon dioxide • SiO2Silica, giant structure similar to diamond, quarts • Silicates, NaSiO4, tetrahedrical, silicon-oxygen single bond
4.3 Intermolecular forces • Holds molecules together in liquids or solids (No Intermolecular forces between gaseous particles) • Weaker than covalent and ionic bonds • Hydrogen bond (Quite strong) • Dipole-dipole (Middle weak) • van der Waal’s forces (~ London dispersion forces) (Very weak) • Accounts for differences in aggregatio state and solubility
van der Waal’s forces • “Vibrations” in the electron cloud => Temporary dipoles. • A temporary dipole in one molecule can induce a temporary dipole in another molecule • Exist between all molecules
van der Waal’s forces, cont. The strength increases with molar mass of the molecule/atom E.g. He b.p 4 K Xeb.p. 165 K Only effective over short range so the molecule “area” is also important. E.g: Pentane, C5H12, b.p. 309 K Dimethylpropane, (CH3)4C b.p. 283 K
Dipole-dipole bond • Electrostatic attraction between molecules with permanent dipoles • Stronger than van der Waals bond Hydrogen chloride M= 36,5 g/molb.p. 188 K Fluorine M= 38 g/molb.p. 85K
Hydrogen bond • In molecules that contain Hydrogen bonded to Oxygen, Nitrogen or Fluorine (high electronegativity and non-bonding electron pair) • Stronger than dipole-dipole bonds • Interaction of the non-bonding electron pair in one molecule and hydrogen (with high positive charge) in another molecule.
Whichintermolecular bond? • H2O b.p.= 100oCH2S b.p.= -61oC • NH3b.p.= -33oCPH3b.p.= -88oC